To avoid dangerous climate change, the growth of atmospheric concentrations of carbon dioxide must be halted, and may have to be reduced. The concentration of carbon dioxide, the most important greenhouse gas, has increased from about 280 ppm in the preindustrial age to more than 385 ppm and it is now increasing by more than 2 ppm per year driven by global CO2 emissions that are now increasing at more than 3.3% per year (Canadell et al., 2007).
Carbon capture and storage (CCS) technologies target CO2 removal from large fixed-point sources such as power plants. Dispersed sources, however, emit more than half of global CO2 emissions. Direct capture of CO2 from ambient air, “air capture”, is one of the few methods capable of systematically managing dispersed emissions. Therefore, while air capture is more expensive that capture from large point sources it remains important as it will primarily compete with emission reductions from dispersed sources such as transportation which can be very expensive to mitigate.
1.1 Air Capture
Carbon dioxide absorption from atmospheric air using alkaline solution has been explored for half a century (Spector and Dodge 1946, Tepe and Dodge 1943). Large scale scrubbing of CO2 from ambient air was first suggested by Lackner in the late 1990's (Lackner et al., 1999). In wet scrubbing techniques, CO2 is absorbed into a solution of sodium hydroxide, NaOH, and is leaving behind an aqueous solution of sodium hydroxide and sodium carbonate, Na2CO3. For this process, the contactor, as the component of the system that provides the contacts between CO2 and sodium hydroxide, has thus far been a point of contention. Large convective tower (Lackner et al., 1999), and packed scrubbing towers (Baciocchi et al., 2006 and Zeman, 2007) are the most commonly suggested contactor designs. A packed tower equipped with Sulzer Mellapak has been proposed by Baciocchi et al. (2006) to absorb CO2 from air with an inlet concentration of 500 ppm to an outlet concentration of 250 ppm using a 2M NaOH solution.
An alternative strategy, suggested by Stolaroff et al. (2007), is to generate a fine spray of the absorbing solution for providing large surface to the air flow through an open tower. This strategy could have the potential to operate with a small pressure drop in air and avoids the capital cost of packing material. Stolaroff et al. (2007) studied the feasibility of a NaOH spray-based contactor by estimating the cost and energy requirement per unit CO2 captured. Water loss, as a major concern in this design, was addressed and it was found that the water loss could be managed by adjusting of the NaOH concentration with temperature and humidity of air, i.e. the higher the concentration of sodium hydroxide, the lower is the water loss, e.g. using ˜7.2M NaOH, at 15° C. and 65% relative humidity, water loss is eliminated.
Conventional scrubbing towers may be filled with structured packing, and a flow of gas that is counter-current to the drainage of liquid through the structured packing is employed.